The present invention relates to a pickup device which is best adapted to pickup high quality audio signals transmitted through bones with a minimum level of noise and without causing howling.
Pickup devices for picking up vibration transmitted through bones of the type in which a piezo-electric element which is supported like a cantilever picks up audio signals transmitted through bones have been well known in the art. For instance, Japanese Patent Publication No. 39763-1978 discloses a pickup device of the type described above. When one inserts such a pickup device into an ear and speaks, the pickup device picks up noise when the surrounding noise level is in excess of 90 dB (A) so that the audio signal cannot be distinctly distinguished from noise. Furthermore, when this pickup device is connected to a loudspeaker, a howling noise is heard very frequently because the audio frequency range includes resonance frequencies of a few orders so that noise whose frequency is close to such resonance frequencies is picked up.
In order to solve the above and other problems encountered in the prior art pickup devices, there has been proposed a method for using a low-pass filter that transmits frequencies lower than the resonance frequencies. However, it is extremely difficult to attenuate only the sensitivity at the resonance frequencies without attenuating the sensitivity of the audio frequency range because the sensitivity of a piezo-electric element which is supported like a cantilever has a high degree of sensitivity to a resonance frequency.
The inventor has proposed the use of a notch filter in order to solve the above-described problem. The use of a notch filter is very effective in decreasing only the peak value of a resonance frequency, but high volume production cannot be attained because pickup devices have their own unique resonance frequencies so that the notch frequency of each notch filter must be matched with the resonance frequency of each pickup device.